1. Field of the Invention
The present invention relates to a method and an apparatus for processing a video signal and a recording medium which are arranged to reliably protect a copyright of a high-quality video signal obtained from a video disk, a digital VCR, or a receiving unit for a digital broadcast.
2. Description of the Related Art
With advance of digital technology, people are likely to easily obtain a high-quality video signal at home. For example, a pre-recorded tape (video tape package) as well as a digital broadcasting receiving terminal and a digital video disk supply a high-quality video signal.
The supplier of the high-quality video signal adds a copyguard signal to the video signal for protecting the copyright of a program represented by the video signal. This copyguard signal inhibits recording of the program with a video tape recorder (VTR). For example, the copyguard signal employs two kinds of systems called a pseudo horizontal synchronous pulse and a color stripe supplied by the Macrovision Corporation.
For realizing a copyguard function, as shown in FIG. 1, the system called the pseudo horizontal synchronous pulse is arranged to insert a pseudo horizontal synchronous pulse in the vertical blanking interval of the video signal for putting an AGC (Automatic Gain Control) circuit of the VTR to an erroneous operating state and thereby degrading the image quality to an impracticable level. For the same purpose, as shown in FIG. 2, the system called the color stripe is arranged to invert a phase of 2 to 4 lines of a color burst signal at every 20 lines, for example, for inverting the color of the image reproduced by the VTR and thereby degrading the image quality to an impracticable level.
By the way, all the influence of the copyguard signal for inhibiting to record the high-quality video signal at home is disallowed to be assured in the TV market. It means that the copyguard signal may interfere with the market of a certain TV set.
That is, in general, the TV set provides a relatively large time constant and a narrow frequency response range of an APC (Automatic Phase Control) circuit used for forming a carrier wave for color demodulation based on a color burst signal. If, therefore, the color stripe system inverts the phase of the four consecutive lines of the color burst signal at every about 20 lines, it gives no influence to the color demodulation. However, some TV sets may provide a small time constant and a wide frequency response range of the APC circuit. In those TV sets, the hue-inverted band appears on the screen.